reduce memory usage

create_hash allocates default_hash_size=65521,
and that is too much for the following variables for
switch implementation.

in messenger.c
 - receive_queues : key is service_name
 - send_queus : key is serviece_name
 - context_db : not used

active transaction id with hash size 16 is small.

in openflow_switch_interface.c
 - contexts : key is xid
Merge pull request #74 from iHiroakiKawai/hash_size

reduce memory usage